The present invention relates generally to earthmoving equipment, such as a bulldozer, and, more particularly, to a crawler vehicle transmission-steering assembly.
As is known, crawler vehicles comprise a pair of lateral tracks powered by a transmission comprising an input shaft driven by a hydraulic or other type of motor; two opposite, coaxial output shafts connected to respective track drive wheels; and two identical lateral reduction units, each interposed between the input shaft and a respective output shaft.
Each output shaft normally is connected to a service brake and clutch, which are controlled independently by a control unit to steer the vehicle. More specifically, steering is effected by disconnecting one of the drive wheels from the transmission by means of the associated clutch, and by braking the disconnected drive wheel while continuing to transmit torque to the other.
A major drawback of known crawler vehicle steering systems of the type described above lies in the steering control turning the vehicle sharply and producing uncontrolled lateral jerking, mainly on account of one of the tracks, when steering the vehicle, being disconnected from the motor or engine.
To eliminate the above drawback, a transmission-steering assembly without the two clutches is used, and which comprises a differential train connecting the two lateral reduction units and which is powered by a further hydraulic motor to vary the relative speed of the two tracks, possibly in conjunction with the braking action of the two service brakes on the respective output shafts.
The above known assembly defines a continuous differential or so-called power-steering system by which the drive wheels are connected permanently to the input shaft, thus enabling smooth, accurate steering of the vehicle.
Such an assembly, however, is unsuitable for small crawler vehicles with a power of e.g. less than 100 kW. Indeed, comprising an extremely large number of component parts, a transmission-steering assembly of the above type is relatively expensive and, above all, bulky, particularly along the output shaft axis.
It is therefore an object of the present invention to provide a crawler vehicle transmission-steering assembly designed to provide a straightforward, low-cost solution to the aforementioned drawbacks.
According to a first aspect of the present invention, there is provided a transmission-steering assembly for a crawler vehicle comprising two lateral tracks; the assembly having a transmission axis, and comprising a propeller shaft extending along said transmission axis; two output shafts extending along said transmission axis and each for driving a respective said lateral track; two reduction units, each interposed between said propeller shaft and a respective said output shaft; power steering apparatus interposed between said output shafts to vary the relative angular speed of the output shafts; and braking device for braking said output shafts.
The transmission-steering assembly is characterized in that said braking device comprise a single service brake exerting, in use, a braking action on said propeller shaft.
According to a second aspect of the present invention, there is provided a crawler vehicle having a transmission-steering assembly with the characteristics as presented hereabove.
These and other objects, features and advantages are accomplished according to the instant invention in which a transmission-steering assembly for a crawler vehicle has a propeller shaft and two output shafts extending along a transverse axis of the vehicle. The two output shafts drive respective tracks of the vehicle, and are driven by the propeller shaft via the interposition of respective identical reduction units. A power-steering device is interposed between the output shafts to vary the relative angular speed of the output shafts. The assembly has a single service brake for exerting, in use, a braking action on the propeller shaft, and a single auxiliary parking brake associated with the steering device.